The present invention relates to improvements in an automatic transmission with an anti-creep control apparatus for a vehicle.
Japanese Patent Provisional Publication No. 63-106449 discloses an apparatus that executes an anti-creep control for preventing a transmitting of engine power to driven wheels during an idling condition and a hill-hold control for preventing the backward movement of a staying vehicle on an uphill road. The shift change control can be executed by selectively operating friction elements, as shown by a time chart in FIG. 10: (a) starting the anti-creep control at the moment T1, (b) starting the hill-hold control at the moment T3 so as to engage a brake for shifting (second coast brake), (c) starting a starting operation of the vehicle at the moment T3, and (d) canceling the anti-creep control by the execution of the starting operation. The apparatus executes a control for starting the engagement of the forward clutch and for promptly disengaging a shifting brake of the automatic transmission at the moment T5 when the hydraulic pressure of a forward clutch (C1) reaches a predetermined value P.sub.sset. During this period, by applying a signal pressure Ps to a servo hydraulic pressure control of the forward clutch and a servo hydraulic pressure control of a brake for shifting, the hydraulic pressure of the both servos are relationally controlled.
However, an actual clutch hydraulic pressure and an actual brake hydraulic pressure are not applied to the servo control of the forward clutch and the shifting brake although the signal hydraulic pressure Ps is used to relationally control the forward clutch and the shifting brake. Therefore, if the responsibility of the actual clutch hydraulic pressure and the actual brake hydraulic pressure are deviated, various problems are generated by this apparatus. For example, in order to start the engagement of the forward clutch (C1) and to simultaneously cancel the anti-creep control at the moment T5 after the acceleration ON at the moment T4 shown in FIG. 10, it is necessary to raise the forward clutch hydraulic pressure to a value that is greater than the hydraulic pressure value Px since the input torque of the automatic transmission is increased. In order to prevent the engagement shock, the hydraulic pressure is once increased to the shelf pressure and then to the maximum pressure. Herein, if the forward clutch hydraulic pressure is radically (such as steppingly) increased to the shelf pressure Px to execute the engagement of the forward clutch, the shifting brake (B1) is fully engaged at the moment T5 and the reduction speed ratio is set at the first speed since the hydraulic pressure of the shifting brake is promptly decreased. As a result, the shock due to the engagement of the forward clutch (C1) is amplified by the first speed and performs as an output shaft torque. Therefore, a drive feels large engagement shock.
Further, when it is intended to radically increase the forward clutch hydraulic pressure, the hydraulic pressure responsibility is largely degraded if entrained air exists in the hydraulic passage of the control hydraulic system for the forward clutch and the shifting brake or if it is cold. When the responsibility of the forward clutch hydraulic pressure is degraded by the entrained air in the hydraulic passage and that no air is mixed in the hydraulic passage of the control hydraulic system of the shifting brake (B1), the hydraulic pressure of the shifting brake (B1) is promptly decreased and the shifting brake (B1) is fully disengaged. Therefore, the hill-hold function becomes inoperative. If the vehicle is put in this situation, during a time period from the moment T4 to the moment T5, the forward clutch hydraulic pressure is not sufficiently increased and the output torque is still small. Therefore, the automatic transmission is temporarily set in the neutral position. This invites a problem that the vehicle is moved backward on an uphill road.